# CBR and K-value to Modulus of Elasticity (E) conversion

For existing pavements originally designed using the California Bearing Ratio (CBR) method, the subgrade modulus can be determined from direct soil testing or using the available CBR values and converting them to an equivalent Modulus of Elasticity using the relationship that appears on the "Aerodrome Design Manual, Part 3 — Pavements Third Edition, 2022".

In rigid pavement design, the foundation modulus can be defined as either the subgrade reaction modulus (k) or the elastic modulus (E). However, all structural calculations are done using the elastic modulus (E). If the foundation modulus is provided as a k value, it can be converted to an equivalent E value.

E (psi) = 20.15 * k^{1.284}

## Subgrade Categories

The Aircraft Classification Rating (ACR) index is defined for 4 standard subgrade categories. These are identified as High, Medium, Low, and Ultra Low, and are assigned the following values of Elastic Modulus. These categories are the same for both rigid and flexible pavements.

Category A (High) | Category B (Medium) | Category C (Low) | Category D (Ultra-low) | |
---|---|---|---|---|

Modulus of Elasticity (MPa) | E ≥ 150 | 100 ≤ E <≤> 150 | 60 ≤ E < 100 | E < 60 |